LeetCode-Easy020-Valid-Parentheses

Easy 020 Valid Parentheses

Description:

“()” “[]” “{}”三种括号匹配问题，判断参数字符串是否满足匹配要求
如：“({[]})” true “[{})” false
Note:空串为true

My Solution:

1. • 括号匹配问题是栈的典型应用，遇到左括号，入栈，遇到右括号，看栈顶是否是相应的左括号，若不是，则false
   • 时间复杂度O(n)
   • 代码如下：

   •     public boolean isValid(String s) {
          if(s.length()==0){
              return true;
          }
          Stack<Character> stack = new Stack<>();
          for(char c:s.toCharArray()){
              if(c == '(' || c == '{' || c == '['){
                  stack.push(c);
              }else if(stack.size()==0){
                  return false;
              }
              else if(c == ')' && stack.peek() == '('){
                  stack.pop();
              }else if(c == ']' && stack.peek() == '['){
                  stack.pop();
              }else if(c == '}' && stack.peek() == '{'){
                  stack.pop();
              }else
                  return false;
          }
          if(stack.size() == 0){
              return true;
          }
          return false;
      }

Fast Solution: